The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
In the constant quest to improve, i.e., reduce, fuel consumption, every aspect of the powertrains of consumer motor vehicles such as passenger cars and light trucks has come under scrutiny. Obviously, the two components most significantly affecting fuel consumption are the engine and the transmission. While such efforts with regard to the engine revolve around control of fuel flow, valve timing, spark timing and combustion techniques to name the more significant, those relating to the transmission involve gearing, shifting and both the electronic and hydraulic control systems.
In the past, automatic transmissions utilized a hydraulic pump driven by the transmission input shaft which supplied transmission (hydraulic) fluid under pressure first to the various control valves and thence to the actuators which effect clutch and brake operation. While mechanically convenient, pumps driven by the drive line had the drawback of operating only when the engine did. Thus, this arrangement was generally unsuitable for engine start-stop (ESS) operation since without additional components, such as an accumulator, pressure could drop below operating minimums during the engine stop phase and such reduced pressure could interfere with vehicle launch after engine restart. Also, the pump would necessarily be sized to provide an assured minimum flow when the engine was idling. Such sizing, of course, would be greatly in excess of the size needed to provide sufficient flow at cruising speed—a condition that might exist for hours at a time during which there was little or no transmission shift activity requiring hydraulic fluid pressure.
One solution to this conundrum has been the utilization of an electric motor to power the transmission hydraulic pump. Such an electric motor driven hydraulic transmission pump can, of course, continue to operate during the stop portion of an engine start-stop cycle. Furthermore, electrical power to the motor can be controlled to increase fluid output during acceleration and deceleration to provide sufficient fluid during periods of high clutch and brake actuator activity and controlled to reduce fluid output during steady state operation.
This solution, however, is not without its own issues. Since the pump now includes an electric motor, during high speed operation it will generate significant heat. Moreover, its output must be controlled and directed to the transmission components most requiring it at any given moment without sacrificing operational stability and repeatability. This and other operational issues are addressed by the present invention.